1. Field of the Invention
The present disclosure generally relates to image forming and, more particularly, to an image forming apparatus having a function of measuring the color of a measurement image.
2. Description of the Related Art
An image forming apparatus has image qualities such as granularity, in-plane uniformity, character quality, and color reproducibility (including color stability). With the proliferation of multicolor image forming apparatuses today, the color reproducibility is sometimes said to be the most important image quality.
Humans have memories of empirically expected colors (of skin, blue sky, and metals in particular), and can get an unconformable feeling if the allowable ranges are exceeded. Such colors are called memory colors, reproducibility of which has often been demanded when outputting photographs.
Aside from photographic images, office users may feel unconformable about a color difference of document images from display monitors. Graphic arts users are pursuing color reproducibility of computer graphics (CG) images. The demand for the color reproducibility (including stability) of image forming apparatuses is thus ever increasing.
To meet the users' demand for the color reproducibility, an image forming apparatus that reads measurement images formed on a sheet with color sensors arranged on a sheet conveyance path has been discussed (for example, see Japanese Patent Application Laid-Open No. 2004-086013).
The image forming apparatus forms the measurement images on a sheet, and gives feedback to a process condition such as the amount of exposure and a developing bias based on the reading result of the measurement images by the color sensors. This enables reproduction of a certain density, graduation, and color tone.
However, according to Japanese Patent Application Laid-Open No. 2004-086013, the color sensors are arranged on a conveyance path near a fixing unit. This arouses concern about a phenomenon called “thermochromism,” where the measurement images to be measured vary in color with temperature. Such a phenomenon occurs because molecular structures forming color materials such as toner and ink are changed by “heat.”
To measure the measurement images inside the image forming apparatus, the color materials need to have been placed on the sheet and the color mixing needs to be completed. If the image forming apparatus uses inks as the color materials, a drying unit needs to heat and dry the measurement images before the measurement. If the image forming apparatus uses toners as the color materials, a fixing unit needs to heat and melt the toners for color mixing before the measurement. The color sensors therefore need to be arranged downstream of the drying unit or fixing unit in the conveyance direction of the sheet.
On the other hand, to constitute the image forming apparatus in a compact configuration, the conveyance path from the drying unit or fixing unit to the color sensors needs to be kept to a minimum necessary length. As a result, the sheet and the color materials heated by the drying unit or fixing unit are conveyed to the color sensors without being cooled to ordinary temperature. Sheet conveyance guides and other internal members of the image forming apparatus and the atmosphere inside also rise in temperature. This also contributes to a sheet temperature higher than ordinary temperature.
As described above, an image forming apparatus including color sensors inside can produce a measurement result different from the color in a normal environment (ordinary temperature environment) because of thermochromism.